The automobile that runs on fuel such as gasoline is equipped with a fuel tank for storing fuel, and an apparatus for determining a remaining amount of fuel, which apparatus is provided so that a driver can be informed of the amount of fuel remaining in the fuel tank.
A conventional apparatus for determining a remaining amount of fuel includes a fuel tank level sensor provided in a fuel tank, and a fuel gauge that indicates the remaining amount of fuel to a driver. The fuel tank level sensor includes a float that floats on the surface of liquid fuel, a contact that moves vertically according as the float moves vertically, and a rheostat that changes an electric resistance value thereof according to a contact position thereof with the contact. Typically, the contact and the rheostat in operation are always put in a state of immersion in liquid fuel or exposure to evaporated fuel.
Conventionally, the fuel tank level sensor determines the amount of fuel remaining in the fuel tank by making use of changes in electric resistance value of the rheostat which take place according as the contact position between the contact and the rheostat changes with vertical movement of the float resting on the surface of liquid fuel.
There is shown in graph form in FIG. 9 a relationship between resistance values of the rheostat and amounts of fuel remaining in the fuel tank. In the graph, the rheostat is configured to let its resistance value decrease linearly according as the amount of fuel remaining in the fuel tank increases.
Incidentally, it is known that fuels such as gasoline contain sulfur which is deposited in sulfide form with time on the contact and rheostat, to raise the resistance value of the rheostat and widen variations in the resistance values.
Consequently, when sulfides are deposited, as indicated by a solid line in FIG. 10, the resistance values of the rheostat are raised overall from early-stage values (a broken line in FIG. 10), and the variations become wide. To be more specific, as the example shown in FIG. 10 demonstrates, values given as an indication of the remaining amounts of fuel by the fuel gauge (readouts) become smaller than actual remaining amounts of the fuel (true values).
This phenomenon is illustrated by FIG. 11. In FIG. 11, the axis of ordinates represents the readouts of the fuel gauge, and the axis of abscissas represents the true values indicating the amounts of fuel actually remaining in the fuel tank. In the early-stage values (as indicated by broken line), good agreement is observed between the readouts and the true values. However, when sulfides are deposited and the resistance value is raised (as indicated by solid line), the agreement is broken and the readouts become smaller than the true values.
Even when the phenomenon as shown in FIG. 11 takes place, a full-tank indication range represented by dash-and-dot lines in FIG. 11 is a range of readouts of fuel levels which may be recognized as a fill-up by a person who visually sees the indication of the fuel gauge. It is shown in FIG. 11 that even if the true value becomes a value equivalent to a fill-up after the sulfides are deposited, the readout still does not reach the full-tank indication range.
Such an undesirable gap between the readouts and the true values, which results from deposition of sulfides as described above, would disadvantageously give a vague sense of injustice to a driver who has made a fill-up request at a gas station or the like. Because the fuel gauge provides a readout that appears as if the fuel tank had not been filled up notwithstanding the fuel tank has actually been filled up with fuel.
The present invention has been made with consideration given to the above-described disadvantages, and it is an exemplified object of the present invention to provide a fuel tank level sensor that can always detect a fill-up of a fuel tank adequately even if a resistance value of a rheostat increases with deposition of sulfides.
Another exemplified object of the present invention is to provide an apparatus for determining a remaining amount of fuel that can precisely determine the amount of fuel remaining in a fuel tank based upon fill-up information detected by the fuel tank level sensor without relying on the resistance value of the rheostat.